Since Rk. II is a faction vendor spell, then Rk. I must be a dropped spell. I have no clue for a specific mob, but since it's level I'd guess, BMK or HoS, or higher. It's probably from a specific mob / mob-type, as most Rk. I's that are dropped come from particular mobs.
You can probably narrow it down, by looking at where the other known Rk. I's drop, and avoidign those mobs / areas, in the higher SoF zones.
Or you could do the faction missions, and buy the Rk. II, or work on the various missions to gain access to the Rk. IIIs. Ladies of the Light is the faction for the Rk. II spell, probaably need Ally with a level 80 spell.
Yther Ore.